1. Field of the Invention
The present invention generally relates to the field of data communications and processing and, more particularly, to a method for synchronizing a CDMA receiver to a transmitter when an adaptive antenna is utilized to receive transmitted data.
2. Description of the Related Art
Code Division Multiple Access (CDMA) modulation is a multi-user access transmission scheme in which different users of the same transmission medium overlap both in frequency and in time. This is in contrast to Frequency Division Multiple Access (FDMA) in which users overlap in time, but are assigned unique frequencies, and Time Division Multiple Access (TDMA) in which users overlap in frequency, but are assigned unique timeslots. According to CDMA, each user is assigned a unique code sequence that allows the user to spread its information over the entire channel bandwidth, as opposed to particular sub-channel(s) in FDMA. Thus, signals from all users are transmitted over the entire channel. To separate out the signals for a particular user at a receiver, cross correlation is performed on the received signal using the same unique user code sequence.
CDMA transmission is well known to those of skill in the art. A comparison between CDMA and FDMA/TDMA may be found in Proakis, “Digital Communications”, Chapter 15, which is incorporated herein by reference. Also, an example of a combined approach for minimizing inter-user interference (i.e., combining a Walsh basis within a group and a spreading sequence across groups) is the IS-95 system described in TIA/EIA/IS-95 “Mobile Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System”, which is incorporated herein by reference.
An IS-95 CDMA system is unique in that its forward and reverse links (i.e., the base station to mobile station and mobile station to base station) have different link structures. This is necessary to accommodate the requirements of a land-mobile communication system. The forward link consists of four types of logical channels, i.e., pilot, sync, paging, and traffic channels, with one pilot channel, one sync channel, up to seven paging channels, and several traffic channels. Each of these forward-linked channels is first spread orthogonally by its Walsh function, and then spread by a pair of short PN sequences (so-called pseudonoise) each of which is a sequence of high data rate bits (“Chips”) ranging from −1 to +1 (polar) or 0 to 1 (non-polar). Subsequently, all channels in the system are added together to form the composite spread spectrum signal which is transmitted on the forward link.
The reverse link in the IS-95 CDMA system consists of two types of logical channels, i.e., access and traffic channels. Each of these reverse-link channels is spread orthogonally by a unique long PN sequence; hence each channel is recovered or decoded using the distinct long PN code. In some instances, a pilot channel is not used on the reverse link based on the impracticality of each mobile station broadcasting its own pilot sequence. Additionally, the IS-95 CDMA system uses 64 Walsh functions which are orthogonal to each other (i.e., their cross-product is equal to zero), and each of the logic channels on the forward link is identified by its assigned Walsh function. The Walsh function is used to generate a code which is used to separate individual users occupying the same RF band to avoid mutual interference on the forward link. The access channel is used by the mobile station to communicate with the base station when a traffic channel is not assigned to the mobile station. The mobile station uses the access channel to generate call originations and respond to pages and orders. The baseband data rate of the access channel is fixed at 4.8 Kilobits per second (Kbps).
The pilot channel is identified by the Walsh function 0 (ω0). This channel contains no baseband sequence information. The baseband sequence is a stream of 0s which are spread by Walsh function 0, which is also a sequence of all zeros. The resulting sequence (still all 0s) is then spread or multiplied by a pair of quadrature PN sequences. Therefore, the pilot channel is effectively the PN sequence itself. The PN sequence with a specified offset uniquely identifies the particular geographical area or sector from which the user is transmitting the pilot signal. In an IS-95 CDMA system, both Walsh function 0 and the PN sequence operate at a rate of 1.2288 mega chips per second (Mcps). After PN spreading, baseband filters are used to shape the resultant digital pulses. These filters effectively lowpass filter the digital pulse stream and control the baseband spectrum of the signal. As a result, the signal band possesses a sharper roll-off near the band edge. The pilot channel is transmitted continuously by the base station sector. The pilot channel provides the mobile station with timing and phase reference. The measurement of the signal-to-noise ratio of the pilot channel by the mobile station also provides an indication of the strongest serving sector of that mobile. Here, the signal-to noise is the energy per chip per interference density, or Ec/I0, where Ec is the energy per chip and I0 is the interference density.
Unlike the pilot channel, the sync channel carries baseband information. The baseband information is contained in the sync channel message which notifies the mobile of information concerning system synchronization and parameters. Similar to the sync channel, the paging channel also carries baseband information. However, unlike the sync channel, the paging channel transmits at a higher rate, i.e., at either 4.8 or 9.6 Kbps.
The forward and reverse traffic channels are used to transmit user data and voice; signaling messages are also sent over the traffic channel. The structure of the forward traffic channel is similar to that of the paging channel, while the structure of the reverse traffic channel is similar to that of the access channel. The only difference is that the forward traffic channel contains multiplexed power control bits (PCBs) and the reverse traffic channel contains a data burst randomizer which is used to generate a masking pattern of 0s and 1s to randomly mask redundant data.
The techniques for separating signals in time (i.e., TDMA), or in frequency (i.e., FDMA) are relatively simple ways of ensuring that the signals are orthogonal and noninterfering. However, in CDMA, different users occupy the same bandwidth at the same time, but are separated from each other via the use of a set of orthogonal waveforms, sequences, or codes. Two real-valued waveforms x and y are said to be orthogonal if their cross correlation Rxy over time period T is zero, where
                                          R            xy                    ⁡                      (            0            )                          =                              ∫            0            T                    ⁢                                    x              ⁡                              (                t                )                                      ⁢                          y              ⁡                              (                t                )                                      ⁢                                                  ⁢                          ⅆ              t                                                          (                  Eq          .                                          ⁢          1                )            In discrete time, the two sequences x and y are orthogonal if their cross-product Rxy(0) is zero. The cross product is defined as
                                          R            xy                    ⁡                      (            0            )                          =                                            x              T                        ⁢            y                    =                                    ∑                              i                =                1                            I                        ⁢                                                  ⁢                                          x                1                            ⁢                              y                i                                                                        (                  Eq          .                                          ⁢          2                )            In this case, T denotes the vector transpose, i.e., a column represented as a row or vice versa. For example, the following two sequences or codes, x and y are orthogonal:xT=[−1 −1 1 1]yT=[−1 1 1 −1]because their cross-correlation is zero; that isRxy(0)=xTy=(−1)(−1)+(−1)(1)+(1)(1)+(1)(−1)=0  (Eq. 3)In order for the set of codes to be used in a multiple access scheme, additional properties are required. That is, in addition to the zero cross-correlation property, each code in the set of orthogonal codes must have an equal number of 1s and −1s. This property provides each particular code with the required pseudorandom characteristic. An additional property is that the dot product of each code scaled by the order of the code must equal to 1. The order of the code is effectively the length of the code, and the dot product is defined as a scalar obtained by multiplying the sequence by itself and summing the individual terms. This is given by the following relationship:
                                          R            xx                    ⁡                      (            0            )                          =                                            x              T                        ⁢            x                    =                                    ∑                              i                =                1                            I                        ⁢                                                  ⁢                                          x                i                            ⁢                              x                i                                                                        (                  Eq          .                                          ⁢          4                )            
In a CDMA system, a specified segment of each sequence available to a node of the network is designated as a “symbol”. In the case of a repetitive sequence, a symbol may be a complete period of the sequence. The time interval during which a node transmits or receives such a symbol is called a “symbol interval”. In a multi-node spread-spectrum network employing multiple orthogonal sequences, all the nodes may simultaneously transmit and/or receive information-bearing symbols derived from some or all of the sequences available to the nodes.
The increasing use of wireless telephones and personal computers has led to a corresponding demand for such advanced telecommunications techniques as CDMA, FDMA and TDMA, which were once thought to be only meant for use in specialized applications. In the 1980's wireless voice communication became widely available through the cellular telephone network. Such services were at first typically considered to be the exclusive province of the businessman because of high subscriber costs. The same was also true for access to remotely distributed computer networks, whereby until very recently, only business people and large institutions could afford the necessary computers and wireline access equipment. As a result of the widespread availability of both technologies, the general population now increasingly wishes to not only have access to networks such as the Internet and private intranets, but also to access such networks in a wireless manner as well. This is of particular concern to the users of portable computers, laptop computers, hand-held personal digital assistants and the like who prefer to access such networks without being tethered to a telephone line.
However, there is still no widely available satisfactory solution for providing low cost, broad geographical coverage, high speed access to the Internet, private intranets, and other networks using the existing wireless infrastructure. This situation is a result of several factors. For one, the typical manner of providing high speed data service in the business environment over the wireline network is not readily adaptable to the voice grade service which is available in most homes or offices. Additionally, such standard high speed data services do not lend themselves well to efficient transmission over standard cellular wireless handsets. Furthermore, the existing cellular network was originally designed only to deliver voice services. As a result, the emphasis in present day digital wireless communication schemes lies with voice, although certain schemes such as CDMA do provide some measure of asymmetrical behavior for the accommodation of data transmission. For example, the data rate on an IS-95 forward traffic channel can be adjusted in increments from 1.2 Kbps to up to 9.6 Kbps for so-called Rate Set 1, and for increments from 1.8 Kbps up to 14.4 Kbps for Rate Set 2.
Existing systems therefore typically provide a radio channel which can accommodate maxinum data rates only in the range of 14.4 Kbps at best in the forward direction. Such a low rate data channel does not directly lend itself to transmitting data at rates of 28.8 or even 56.6 Kbps which are now commonly available with conventional modem type equipment. Data rates at these levels are rapidly becoming the minimum acceptable rates for activities such as Internet access. Other types of data networks using higher speed building blocks such as Digital Subscriber Line (xDSL) service are just now coming into use. However, the cost of xDSL service has only recently been reduced to the point where it is attractive to the residential customer.
Although xDSL and Integrate Services Digital Network (ISDN) networks were known at the time that cellular systems were originally deployed, for the most part, there is no provision for providing higher speed ISDN or xDSL grade data services over cellular networks. Unfortunately, in wireless environments, access to channels by multiple subscribers is expensive and there is competition for them. Whether the multiple access is provided by the traditional FDMA using analog modulation on a group of radio carriers, or by the newer digital modulation schemes which permit sharing of a radio carrier using TDMA or CDMA, the nature of the radio spectrum is that it is a medium which is expected to be shared. This is quite different from the traditional environment for data transmission, in which the wireline medium is relatively inexpensive to obtain, and is therefore not typically intended to be shared. Accordingly, it is apparent that there is a need to provide a system which supports higher speed ISDN or xDSL grade data services over cellular network topologies. In particular, what is needed is an efficient scheme for supporting wireless data communication such as from portable computers to computer networks such as the Internet and private intranets using widely available infrastructure.
Current wireless standards in widespread use such as CDMA do not provide an adequate structure with which to support the most common activities, such as web page browsing. In the forward and reverse link direction, the maximum available channel bandwidth in an IS-95 type CDMA system is only 14.4 Kbps. Due to IS-95 being circuit-switched, there are only a maximum of 64 circuit-switched users that can be active at one time. In practicality, this limit is difficult to attain, and 20 or 30 simultaneous users are typically active at one time. Furthermore, existing CDMA systems require certain operations before a channel can be used. For example, both access and traffic channels in such systems are modulated by PN sequences. As a result, a search of all possible PN offsets is required for synchronization of the receiver with the transmitter. This is due to the existence of 32,768 PN offsets associated with each base station which must be searched individually. When an adaptive antenna system is used with the receiver, such a search includes both the PN offsets and an azimuth angle of the antenna. Furthermore, if the antenna steers nulls on the interferers (i.e., sets the azimuth angle of the antenna toward other transmitters), the overall search will also include another search parameter (i.e., the null steering parameter). Such an iterative search over the PN offsets, the azimuth angle and the null steering is a slow and time consuming process which produces a noticeable synchronization delay to a user of a subscriber unit due to the necessity to search over the additional constraints.